Automatic brightness compensation for x-ray imaging systems

Abstract

The brightness of an X-ray video image during fluorography is maintained at a substantially constant level by a control circuit which varies the X-ray dose in relation to changes in the average brightness of the X-ray image. As the X-ray system approaches the limits of its imaging capability, varying the X-ray dose alone may not yield the desired brightness level. At this point, the gain applied to the video signal is increased to improve the brightness. A linear brightness taper function is used such that, as the level of video gain required to maintain constant brightness increases, the actual video gain increases by a smaller proportional amount. This function results in the brightness of the video image decreasing somewhat as the video gain is required to provide a greater degree of brightness compensation. This reduction in brightness not only provides a visual indication to the image observer that the system is approaching the imaging limits, but also creates an illusion that noise artifacts in the image are not intensifying as the video gain increases.

What is claimed is: 
     
       1. In an fluoroscopic imaging system having an X-ray tube that when excited emits an X-ray beam, an apparatus which converts an image produced by the X-ray beam into a video signal and applies video gain to the video signal, and means for displaying a video image from the video signal; the improvement comprising a circuit for controlling the brightness of the video image comprising: means for determining a deviation of the brightness of the video image from a brightness reference level;   means, responsive to said means for determining, for altering the excitation of the X-ray tube to reduce the deviation of the brightness of the video image from the brightness reference value;   means for indicating the degree to which said means for altering the excitation of the X-ray tube is unable to eliminate the deviation of the brightness of the video image from the brightness reference value,   means, responsive to said means for indicating, to produce an indication designated "Demanded Video Gain" representing the video gain necessary in order for the brightness of the video image to equal the brightness reference value; and   means for varying the video gain such that, as the Demanded Video Gain increases, the video gain is varied to decrease the brightness of the video image.   
     
     
       2. The circuit as recited in claim 1 wherein said means for altering the excitation of the X-ray tube includes means for producing a first signal indicating an electron beam current level to be produced in the X-ray tube; and means for producing a second signal indicating a bias voltage level to be applied between an anode and a cathode of the X-ray tube. 
     
     
       3. The circuit as recited in claim 2 wherein said means for altering the excitation of the X-ray tube initially varies the first signal to reduce the brightness deviation; and if varying the first signal alone is insufficient to eliminate the brightness deviation, said means for altering the excitation of the x-ray tube also varies the second signal. 
     
     
       4. The circuit recited in claim 1 wherein said means for varying the video gain includes means for producing a tapered brightness reference value (TBF) given by the linear function: TBF=m (Demanded Video Gain)+b   where m is the slope of the linear function having a negative value and b is a constant.   
     
     
       5. The circuit recited in claim 4 wherein the slope m of the of the linear function is defined by: ##EQU6## and the constant b is defined by: ##EQU7## where BRT1 is the video image brightness produced at a first known value VG1 of Demanded Video Gain, and BRT2 is the video image brightness produced at a second known value VG2 of Demanded Video Gain. 
     
     
       6. The circuit as recited in claim 4 wherein said means for varying the video gain utilizes one of a plurality of predefined sets of values for m and b for the linear function depending upon which one of an equal plurality of X-ray dosages is selected for a given exposure. 
     
     
       7. The circuit recited in claim 4 wherein said means for varying the video gain further includes: means for comparing the brightness of the video image to the tapered brightness reference value; and   means for producing a video gain value in response to said means for comparing the brightness of the video image.   
     
     
       8. The circuit recited in claim 4 wherein said means for varying the video gain does not produce a tapered brightness reference value that is less than a minimum value. 
     
     
       9. The circuit recited in claim 1 wherein said means for varying the video gain does not decrease the brightness of the video image below a minimum level (MIN). 
     
     
       10. The circuit recited in claim 1 wherein the apparatus which converts an image produced by the X-ray beam into a video signal includes a variable optical iris and a variable gain amplifier, and wherein said means for varying the video gain comprises: means for varying the gain of the amplifier until a given gain threshold is reached and thereafter inhibiting further variation of the gain of the amplifier until the optical iris is substantially at a maximum aperture opening; and   means for varying the optical iris after the gain of the amplifier reaches the given gain threshold.   
     
     
       11. The circuit recited in claim 1 wherein the apparatus which converts an image produced by the X-ray beam into a video signal includes a variable optical iris and a variable gain amplifier, and said circuit for controlling the brightness of the image further comprising: means for comparing a desired video gain level to a iris control threshold;   means for altering an aperture size of the iris when said means for comparing indicates that the desired video gain level exceeds the iris control threshold;   means for deriving a value corresponding to video gain level provided by the iris; and   means for varying the gain of the amplifier in response to the difference between the value corresponding to video gain level provided by the iris and the desired video gain level.   
     
     
       12. The circuit as recited in claim 1 wherein said means for varying the video gain utilizes one of a plurality of predefined arithmetic functions to determine a level for the video gain in response to the Demanded Video Gain depending upon which one of an equal plurality of X-ray dosages is selected for a given exposure. 
     
     
       13. In an fluoroscopic imaging system having an X-ray tube that when excited emits X-rays, means for converting an X-ray image into a visible light image, a camera for producing an electrical signal representing the visible light image, means for displaying a video image from the signal; the improvement comprising a circuit for controlling the brightness of the video image comprising: means for deriving an indication of the brightness of the video image;   means for comparing the video image brightness indication to a brightness reference value to determine a deviation from the brightness reference value;   means, responsive to said means for comparing, the video image brightness indication for altering the excitation of the X-ray tube to reduce a deviation of the derived image brightness indication from the brightness reference value;   means for indicating when said means for altering the excitation of the X-ray tube is approaching the limit of the latter means ability to alter the excitation of the x-ray tube;   means, which responds to said means for indicating when the means for altering the excitation of the x-ray tube is approaching the limit, for varying a gain applied to the electrical signal to thereby alter the brightness of the video image so that the brightness of the image decreases as the present means is required to compensate for more of the brightness deviation.   
     
     
       14. The circuit as recited in claim 13 wherein said means for altering the excitation of the X-ray tube includes: a first means for varying an electron beam current of the X-ray tube; and   a second means for varying a bias voltage applied to the X-ray tube;   wherein said first means for varying initially alters the electron beam current to reduce the brightness deviation, but when varying the electron beam current alone is insufficient to eliminate the brightness deviation, said second means for varying alters the bias voltage to further reduce the brightness deviation.   
     
     
       15. The circuit recited in claim 13 wherein said means for varying the gain determines the gain to be applied to the electrical signal utilizing the following relationship: ##EQU8## where the Demanded Gain is the amount of gain that is required to maintain the brightness of the video image at a level defined by the reference value, BRT1 is the video image brightness produced at a first known value DG1 of Demanded Gain, and BRT2 is the video image brightness produced at a second known value DG2 of Demanded Gain. 
     
     
       16. The circuit recited in claim 13 wherein said means for varying the gain does not decrease the brightness of the video image below a minimum level. 
     
     
       17. In an fluoroscopic imaging system having a vacuum tube with a filament, a cathode and an anode that emits an X-ray beam, a converter responsive to the X-ray beam for producing a video signal representing an image produced by the X-ray beam and means for displaying a video image from the signal; the improvement comprising a circuit for controlling the brightness of the video image comprising: means for deriving an indication of the brightness of the video image;   a first means for comparing the image brightness indication to a brightness reference value and producing a first control signal indicative of the relationship of the two compared signals;   means, responsive to the first control signal, for applying a filament current to the vacuum tube to reduce a deviation of the image brightness indication from the brightness reference value;   a second means for comparing the applied filament current to a current reference value and producing a second control signal indicative of the relationship therebetween;   means, responsive to the first and second control signals, for applying a bias voltage across the X-ray tube anode and cathode to further reduce a deviation of the image brightness indication from the brightness reference value;   a third means for comparing the applied X-ray tube bias voltage to a bias voltage limit and producing a third control signal indicative of the relationship therebetween;   a fourth means for comparing the applied filament current to a current limit and producing a fourth control signal indicative of the relationship therebetween;   means, responsive to the first, third and fourth control signals, for generating a fifth control signal indicative of an amount of gain for the video signal that is required to eliminate a deviation of the image brightness indication from the brightness reference value; and   means for applying video gain to the video signal wherein the video gain varies in proportion to the fifth control signal such that the brightness of the video image decreases as the fifth control signal indicates that the video gain must increase to eliminate the specified deviation.   
     
     
       18. The circuit recited in claim 17 wherein said means for applying video gain includes: means for producing a tapered brightness reference value (TBF) given by the linear function: ##EQU9## where the Demanded Video Gain is the level of video gain that is required to maintain the brightness of the video image at the brightness defined by the reference value, BRT1 is the video image brightness produced at a first value VG1 of Demanded video Gain, and BRT2 is the video image brightness produced at a second value VG2 of Demanded Video Gain;   means for comparing the brightness of the video image to the tapered brightness reference value; and   means for producing a video gain value in response to said means for comparing.   
     
     
       19. The circuit recited in claim 18 wherein said means for producing a tapered brightness reference value includes means which prevents the tapered brightness reference value from being less than a minimum level. 
     
     
       20. The circuit for controlling the brightness of the image as recited in claim 18 wherein the converter includes a variable optical iris and a variable gain amplifier, and further comprising: a fifth means for comparing a desired video gain level to an iris control threshold;   means for altering an aperture size of the iris when said fifth means for comparing indicates that the desired video gain level exceeds the iris control threshold;   means for deriving a value corresponding to a video gain level provided by the iris; and   means for varying the gain of the amplifier in response to the difference between the value corresponding to a video gain level provided by the iris and the desired video gain level.   
     
     
       21. In an fluoroscopic imaging system having an X-ray tube that when excited emits an X-ray beam, an apparatus which converts an image produced by the X-ray beam into a video signal which apparatus has a variable optical iris and a variable gain amplifier to apply video gain to the signal, and means for displaying a video image from the signal; the improvement comprising a circuit for controlling the brightness of the video image comprising: means for determining a deviation of the brightness of the video image from a brightness reference level;   means, responsive to said means for determining, for altering the excitation of the X-ray tube to reduce the deviation of the brightness of the video image from the brightness reference value;   means for indicating the degree to which said means for altering the excitation of the X-ray tube is unable to eliminate the deviation of the brightness of the video image from the brightness reference value,   means, responsive to said means for indicating, for producing an indication of a desired video gain to be applied to the signal;   means for producing the desired video gain by adjusting the gain of the amplifier when the gain of the amplifier is below a threshold level, when the gain of the amplifier is adjusted to the threshold level and additional video gain is desired only the optical iris is adjusted until the optical iris is substantially at a maximum aperture opening, at which point if additional video gain is desired the gain of the amplifier is adjusted again.   
     
     
       22. The circuit recited in claim 21 wherein said means for producing an indication of a desired video gain produces an indication which results in the brightness of the video image decreasing as the desired video gain increases in magnitude. 
     
     
       23. The circuit recited in claim 21 wherein said means for producing an indication includes: means for deriving a tapered brightness reference value in response to said means for indicating;   means for comparing the brightness of the video image to the tapered brightness reference value; and   means for generating the indication of a desired video gain value in response to said means for comparing.